The taxonomic position of a novel marine bacterium isolated from the green alga Ulva fenestrata collected in the Sea of Japan was established. Cells of the strain studied, designated KMM 6017T, were strictly aerobic, heterotrophic, pink-pigmented, non-motile by gliding, Gram-negative and oxidase-, catalase-, β-galactosidase- and alkaline phosphatase-positive. 16S rRNA gene sequence analysis indicated that the strain occupied a distinct lineage within the phylum ‘Bacteroidetes’ and formed a cluster with [Flexibacter] tractuosus and Reichenbachia agariperforans. The G+C content of the DNA of KMM 6017T was 40·2 mol%. The major respiratory quinone was MK-7. The predominant fatty acids were i15 : 1, i15 : 0 and i17 : 0 3-OH (34·2, 24 and 7·7 %, respectively). On the basis of phenotypic, chemotaxonomic, genotypic and phylogenetic characteristics, the novel bacterium was assigned to the genus Roseivirga gen. nov., as Roseivirga ehrenbergii gen. nov., sp. nov. The type strain is KMM 6017T (=KCTC 12282T=LMG 22567T).